Firth Brown harness tester

I was given an old Firth Brown 'Hardometer' hardness tester the other day. Sadly it has suffered from years of neglect, most of which can simply be remedied even if it will never have gleaming chrome again. It is short of the guts of the microscope - I reckon I can find a suitable substitute there - and the 1mm hard ball itself is missing. Anyone here know a) proper operating procedure for these things, b) is the plug on the side of the working head, with a fancy chrome tee handle, an oil filler cap or something else? c) Where can I find a

Thanks

Tim